The device is not a bunch of strangers. There's no humans listening to "your" conversation. Though they may be listening to anonymized clips. The activity stored is also your own to control.
I think such privacy concepts are too abstract for a child to be concerned about. You'd be hurting their view of technology and enforcing an idea of Big Brother if you were to teach them a voice assistant is a bunch of strangers listening to you.
>>feocco+(OP)
I think facebook+cabridge analytica et al, and google's collection of location data even when the user opts out (just to name two), shows that it really is a bunch of strangers listening to you.